Cytokines such as interleukin (IL)-2, IL-4 and IL-12 dramatically influence the immune response, regulating both the magnitude and the character of the response. Understanding the molecular basis of this regulation should provide important insights that could shed light on disease pathogenesis and offer new therapeutic targets. In effort to better understand cytokine signal transduction, we cloned a kinase, Jak3, a critical component for signaling by the subfamily of cytokines that share a common receptor subunit, the common gamma chain. These cytokines include: IL-2, IL-4, IL-7, IL-9 and IL-15. We further demonstrated that mutation of Jak3 is the basis for one form of primary immunodeficiency, autosomal recessive severe combined immunodeficiency (SCID). We identified patients who either lack this protein entirely or have missense mutations. We analyzed the consequence of Jak3 deficiency on cytokine signaling and characterized the biochemical function of these mutant missense alleles. Functionally, we demonstrated that loss of Jak3 abrogates IL-2 signaling although Jak3-independent pathways exist for IL-4. We also demonstrated that this was the case with mutation of the common gamma chain which results in the disease X-SCID. In order to test the feasibility of gene therapy for Jak3- and X-SCID, we set up in vitro models to study the reconstitution of signaling. We also studied in detail how Jak3 and the common gamma chain associate and the means by which Jak3 is regulated enzymatically. The importance of Jak3 in coupling IL-2 signaling to the mitogen activated protein kinase pathway was also analyzed and a role for the tyrosine phosphatase, SHP2, in IL-2 signaling was also demonstrated. Detailed structure/function studies are underway to clarify the mechanisms by which Jak3 transmits IL-2 dependent signals. We also sequenced the Jak3 gene and determined its chromosomal localization. IL-12 is an essential cytokine that regulates cell mediated immunity. We have determined that it activates the Janus kinases, Jak2 and Tyk2 and STAT4. Suprisingly, we showed that interfon alpha also activates STAT4, suggesting a role role for it in regulating cell-mediated immunity. Finally, we found that both interferon alpha and IL-12 induce a further modification of STAT4 namely, serine phosphorylation, indicating another commonality in interferon alpha and IL-12. Work is underway to characterize genes that are regulated by both interferon alpha and IL-12 and those that are uniquely regulated by these factors.